Page 335 - Petroleum Geology

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Drilling in Trinidad and the US. Gulf Coast around 1940 led to some
blowouts in which the pore pressures apparently greatly exceeded anything
that could reasonably be explained in terms of a normal hydrostatic water
regime. These difficulties were encountered more widely by the 1950s - in
Brunei, Indonesia, Burma, for example, and then the Niger delta. In the
U.S.S.R. similar experiences occurred (see Fertl, 1976, p. 334). Abnormal
these pressures might be, but they were becoming very common.
Typically, the experience was this: having drilled 1.5 km or so without
difficulty, penetration rate increased and soon afterwards the mud was ob-
served to be flowing from the well at a greater rate over the shale shaker, and
the mud tanks started filling up. When drilling was stopped and the mud
pumps shut down, mud continued to flow from the well at an increasing
rate, so the blowout preventers were closed. It was commonly found that mud
of specific gravity 1.5-1.8 was required to control the influx into the bore-
hole. If drilling was continued, the specific gravity of the mud had to be con-
tinually increased until the practical limit of about 2.2 was reached. The
borehole then had to be abandoned. Such heavy muds were needed to con-
trol pressures that approached those of the total overburden, corresponding
to specific gravities of 2.2-2.4.
In some boreholes, the mudstone tended to squeeze into the hole, making
it tight and tending to stick the tools. These beds were called “heaving shales”.
They showed up on caliper logs, and it was quite common to find that one
could not get back to bottom after a round trip without reaming or redrilling
the last part.
If there was much open hole when the mudweight was increased, there
was an increased tendency for the pipe to stick - not on bottom, but well
up the hole in the normally pressured part of the sequence. This was called
“wall-sticking”. The excess pressure of the mud over the formation fluids held
the drill pipe to the wall of the hole with considerable force. (See Thomeer
and Bottema, 1961, for some histories of drilling abnormal pressures.)
The practical solution of these problems is not without interest. In the
mid-1950s it was found that the best manner of drilling in areas in which ab-
normal pressures were expected was to use the lightest possible mud from the
beginning. This was a remarkable conclusion in the face of the normal prac-
tice of weighting up the mud in anticipation of abnormal pressures. With lighter
mud, the normally pressured part of the hole drilled faster; and careful con-
trol of the drilling parameters resulted in early detection of the increasing
drilling rate (called the “drilling break”).
As soon as the drilling break was detected, the bit was pulled back above
where the drilling break started, pumps were stopped and the mud-level in
the borehole watched. The mud was then circulated “bottoms up”. It could
thus be determined if there was influx into the borehole, and, by analysing
the mud from bottom, the nature of the influx. If the borehole flowed, the
well was said to have kicked; the blowout preventers were closed and the